1. Field of the Invention
This invention relates to electromagnetically controlled shutters.
2. Description of the Prior Art
In the art of electrically operated shutters of the type having the a latch member cooperative with the armature of an electromagnet so that the shutter is latched in the charged position by the attractive force of the electromagnet, and whereas when the armature moves away from the electromagnet as the electromagnet is de-magnetized, the shutter is released, it is generally known that when finely-divided particles or oil drops are adhered to the confronting surfaces of the yoke of the electromagnet and the armature, the attraction of the armature becomes insufficient or impossible and the shutter does not work normally. To avoid this, the armature has to be arranged to return to the initial attracted position as soon after its movement away from the yoke as possible. In the prior art there is known a shutter in which the returning of the armature is controlled by a signal indicating that the trailing curtain of the shutter has run down. With such a shutter, however, when making a long exposure with a slow speed shutter or bulb, the armature is left away from the yoke during the entire exposure time. Therefore, the probability of adhesion of dust or oil is very high. In this respect, it cannot be said that the above-stated aim is fully achieved.
It is also known to provide another arrangement adapted to be used in a single lens reflex camera such that the armature is returned when the drive member for the reflex mirror and automatic diaphragm returns, as disclosed in Japanese Utility Model Application No. Sho 47-34094.
Since, however, in of these arrangements there is no attractive force during the time that the electromagnet is not energized, there is a need for making use of a retaining mechanism for holding the armature in contact with the yoke against the breakaway spring until just before the start of running down movement of the shutter and a reset mechanism responsive to the signal representing the completion of the movement of the trailing curtain for resetting the armature. In addition thereto it is difficult to finely adjust the spring. For these reasons, conventional shutters have drawbacks in that the shutter mechanism becomes complicated with an increase in the space which it occupies, and that the production cost is unavoidably increased.
On the other hand, in a focal plane shutter wherein the curtains are driven to run down by the force of the springs another aspect is involved in that the speed of movement of the shutter curtain is, because of its dependence upon the inertia and friction of the actuator and the curtain itself, slow in the early stage and increases as running goes on. In a focal plane shutter having leading and trailing curtains with such running characteristics, it is difficult to provide assurance that the entire area of the picture frame is uniformly exposed.
To eliminate this drawback, an arrangement may be provided wherein the preliminary movement of the shutter curtain is elongated so that until the slit-forming border of the shutter curtain reaches the exposure aperture, the running apeed is sufficiently raised. In this case, however, a drawback arises in that the space occupied by the shutter increases.